The behavioral and emotional competencies that emerge during the preschool period are predictive of school readiness and long term socio-emotional adjustment. Consequently, this period has been acknowledged as a critical window for intervention. There is an abundance of research indicating that specific temperament characteristics can function as risk or protective factors in child development. Much of this work assumes that temperament represents stable and enduring characteristics and overlooks the possibility that there may be age-related changes in temperament that impact development in other domains. Moreover, there are a number of methodological problems that have hampered research in this area (i.e., reliance on a single rater; measures insensitive to change). The proposed longitudinal, multi-method twin study will address these weaknesses. Latent Growth Models will be used in a genetically-informative twin sample to examine patterns of temperament change across the preschool period and their relations to behavior problems, prosocial behaviors and academic readiness-three behavioral areas that have substantial long-term significance. We will also explore the mechanisms responsible for developmental change, both in terms of parenting effects, and genetic and environmental influences more generally. The temperament dimensions of negative emotionality, positive emotionality, activity level, attention, persistence, shyness and inhibitory control will be longitudinally accessed via multiple methods (e.g., behavioral tests, observations, actigraphs and parent ratings) in a sample of 300 twin pairs at 3, 4, and 5 years of age. Parent report and observational measures of parenting behavior will also be obtained at each age. Developmental outcome measures will include externalizing and internalizing behavior problems, prosocial behavior, and academic readiness. The specific aims of the study are to: 1) Explore individual differences in developmental change across the preschool period using observational measures of temperament in addition to parent ratings; 2) Assess links between child temperament and parenting trajectories. 3) Explore relations between temperament trajectories and developmental outcomes at age 5; and 4) Examine genetic and environmental influences on individual differences in temperament trajectories. The knowledge gained from this study will enhance understanding of temperament development and its relation to children's mental health outcomes and will contribute significantly to early childhood interventions through aiding in the early identification of high risk children, and providing insight on the early malleability of risky versus protective temperament characteristics.